Brakes
Fundamentals
Follow the air path: compressor → governor → air dryer → supply and primary/secondary reservoirs → foot valve → relay valve → brake chambers, with spring (parking) brakes applied by released air pressure. Know typical governor cut-out near 125 psi and cut-in near 100 psi, low-pressure warning by 60 psi, and the pressure-protection valve that preserves supply pressure for the parking brakes.
Adjustment and testing
- Slack adjusters: measure pushrod stroke against the chamber's stamped limit; a standard-stroke type 30 chamber is commonly limited to about 2 inches. Automatic slack adjusters should not be manually backed off to compensate for a worn foundation brake - fix the cause.
- Static leak test: with the system fully charged and the engine off, note pressure drop with brakes released and then applied. Typical published limits are small (a few psi per minute); always compare to the OEM and CVSA criteria.
- Applied stroke and pushrod angle must be within spec, and the chamber must not contact the axle.
Hydraulic side
Know DOT 3/4 glycol versus DOT 5 silicone (not interchangeable), the danger of moisture contamination and boiling, and the function of metering, proportioning, and residual valves.
Traps
Questions offer "adjust the automatic slack adjuster" as a fix for long stroke when the real cause is a worn S-cam bushing or broken return spring. Another trap is a compressor that runs constantly - suspect the governor or dryer purge valve, not the compressor.
Worked example
Cut-out 125 psi, cut-in 100 psi gives a 25 psi cycle. A system cycling 85 to 125 psi has a governor or dryer signal problem, not a failed compressor.